1. Field of the Invention
The present invention relates to improvements in the supported liquid phase alkylation of aliphatic hydrocarbons in the presence of a fluorinated sulphonic acid catalyst.
2. Description of the Related Art
Acid catalyzed alkylation of aliphatic hydrocarbons with olefinic hydrocarbons is a well known process for the preparation of high octane gasoline products. In the past, alkylation of hydrocarbons has been accomplished in liquid phase by mixing paraffins and olefins in the presence of a strong acid catalyst and stirring the mixture until the alkylation reaction was completed.
To date the only employed acid catalysts in the industrial alkylation of aliphatic hydrocarbons are concentrated sulphuric acid or anhydrous hydrofluoric acid, the strength of which may be increased by addition of a Lewis acid, such as BF.sub.3 or SbF.sub.5.
The known acid catalyzed batch processes require large reaction volumes and thorough mixing of the alkylation mixture by mechanical mixing means in order to provide intimate contact between the acid catalyst, the reacting hydrocarbons and the olefinic alkylating agent.
Although being very efficient catalysts a major drawback of the known acid catalysts are their environmental and health risk, when used in large amounts such as in the batch processes.
Beside of being hazardous materials sulphuric acid and hydrofluoric acid are rather unstable or aggressive compounds under the reaction conditions used in the known alkylation processes. At ambient conditions hydrofluoric acid is a volatile gas, which necessitates the alkylation process to be carried out at low temperatures or at elevated pressure. While sulphuric acid is a liquid with a high boiling point and much easier to contain in the event of an accident, it is consumed in considerable amounts during the process by reduction to volatile sulphur dioxide and other unwanted products.
Utilization of fluorinated sulphonic acids, as efficient catalysts during the alkylation of aliphatic hydrocarbons with olefins, is disclosed by the prior application Ser. No. 626,956. Besides being less volatile compounds with appreciatively minor environmental and health risk than hydrofluoric acid, the fluorinated sulphonic acids, when used as alkylation catalysts, are not disintegrated during the alkylation reaction as is the case for sulphuric acid. In essence, the prior application is related to supported liquid phase alkylation of a process stream including a hydrocarbon substrate and an olefinic alkylating agent, by contact with a fluorinated sulphonic acid catalyst in a fixed bed alkylation reactor of polar contact material, in which there is established on the contact material a reaction zone with the fluorinated sulphonic acid catalyst adsorbed within a confined area of the contact material. In the reaction zone the process stream is converted at alkylating conditions to a product stream of alkylated hydrocarbons by catalysis of the fluorinated sulphonic acid adsorbed on the contact material.
During the alkylation reaction, the acid catalyst and, consequently, the reaction zone, move to a new position located nearer the outlet end of the alkylation reactor by interaction with the process stream flowing through and reacting in the zone.
As a theoretical explanation, the elution of the catalyst acid is caused by reactions of the fluorinated sulphonic acid with olefins in the process stream to an ester, which is less polar than the original acid and more loosely adsorbed on the contact material in the reaction zone. The ester moves together with the process stream until it is cleaved to yield the free acid and a carbonium-ion, which reacts with the hydrocarbon substrate to form alkylated hydrocarbons.
The migration speed of the acid catalyst in the reactor and on the contact material is thereby much lower than the migration speed of the hydrocarbons in the process and product stream resulting in a much longer elution time for the acid catalyst than the elution time for the hydrocarbons.
During the migration of the acid catalyst on the contact material, the catalytic activity of the fluorinated sulphonic acid is substantially retained and the acid is still catalytic active, when the reaction zone reaches the reactor outlet.
It is thus possible to reuse the acid catalyst without recovery of the acid, as it reaches the outlet end of the alkylation reactor by reversing the flow direction of the process stream introduced into the alkylation reactor. The reaction zone is then pushed towards the opposite end of the reactor by interaction with the process stream as described above.
Thus, in order to reuse the acid catalyst by the process of the previous application, the acid has to be pushed inside the reactor forth and back on the contact material by periodically reversing the flow direction of the process stream introduced into the reactor.
Despite of its high alkylation efficiency a general drawback of the above process lies in the alternating forth and back flow of the process stream through the contact material. Thereby, a slight fluidization of the contact material may be introduced by the process stream resulting eventually in distortion of the reaction zone by unsymmetric distribution of the acid catalyst in the fluidized contact material. This may further lead to by-pass regions in the reaction zone and thus to diminished alkylation efficiency.